Sen. Daniel Inouye, the longest-serving sitting U.S. Senator, has been hospitalized at Walter Reed National Military Medical Center, to regulate his oxygen intake, his office confirms this afternoon.

"For the most part, I am OK," Sen. Inouye, D-Hawaii, said in a paper statement issued to ABC News, "However, I am currently working with my doctors to regulate my oxygen intake. Much to my frustration, while undergoing this process, I have to remain in the hospital for my own safety and to allow the necessary observation."

On Thursday the 88 year-old senator was at admitted to George Washington hospital, where he remained thru Sunday. He was then transferred to Walter Reed, just outside of Washington, DC.

Inouye said he hopes to be back on Capitol Hill as soon as doctors will allow.

Recently he has been seen using oxygen on the floor of the Senate and has been spotted also using a wheelchair at times.

Inouye's office explained that in the late 1960's, he was misdiagnosed with lung cancer and had a sizeable portion of his left lung removed. After living with this his whole life, his office says about nine months ago his doctors recommended he begin using an oxygen supplement.

Also at the advice of his doctors, the Senator sometimes uses a wheelchair in order to preserve his knees.

"After 50 years of walking the marble floors of the Capitol complex, the marble has taken its toll and he has been advised to avoid long walks on hard surfaces," a spokesperson says.

In November the Senator was taken to Walter Reed after a fall in his apartment and was treated for a minor cut to the back of his head.

Inouye, who served in World War II and won is a Medal of Honor winner, was first elected to the Senate in 1963. He'll start his 50th year in the body Jan. 3.